Conventionally, hydraulic arrangements for parking brakes on agricultural equipment only contain one solenoid valve with two slides. When the solenoid valve is energized to an “ON” position, the piston chamber is pressurized so that the piston is retracted and the parking brake is deactivated. The parking brake can be applied by reducing the pressure in the chamber so that the piston is not restrained.
When the tractor is turned off while in motion or if the tractor suffers a loss of power, a resultant fall in pressure in the circuit occurs. The loss of power to the circuit causes the solenoid valve to deenergize, which allows the oil to return to the drain and reduces the pressure in the chamber. The parking brake is thereby activated in a totally uncontrolled manner and results in sharp highly undesirable braking of the tractor.
A parking brake with a safety device to allow controlled braking during blackout electrical failures or other large pressure drops in the system would be highly desirable.